FIG. 6 is an explanatory view showing an operating principle of a redox flow battery. As illustrated therein, the redox flow battery has a cell 1 separated into a positive electrode cell 1A and a negative electrode cell 1B by a membrane 4 of an ion-exchange membrane. The positive electrode cell 1A and the negative electrode cell 1B include a positive electrode 5 and a negative electrode 6, respectively. A positive electrode tank 2 for feeding and discharging positive electrolytic solution to and from the positive electrode cell 1A is connected to the positive electrode cell 1A through conduit pipes 7, 8. Similarly, a negative electrode tank 3 for feeding and discharging negative electrolytic solution to and from the negative electrode cell 1B is connected to the negative electrode cell 1B through conduit pipes 10, 11. Aqueous solution containing ions that change in valence, such as vanadium ion, is used for the positive and negative electrolytes. The electrolyte containing the ions is circulated by using pumps 9, 12, to charge and discharge the electrolyte with the change in ionic valence at the positive and negative electrodes 5, 6. When the electrolyte containing the vanadium ions is used, the following reactions occur in the cell during the charge and discharge of electricity:
Positive electrode: V4+→V5++e− (Charge) V4+←V5++e− (Discharge)
Negative electrode: V3++e−→V2+ (Charge) V3++e−←V2+ (Discharge)
FIG. 7 is a diagrammatic illustration of construction of a cell stack used for the redox flow battery mentioned above. This type of battery usually uses the construction which is called a cell stack 100 comprising a plurality of cells stacked in layers. Each of the cells has the positive electrode 5 and the negative electrode 6 which are made of carbon felt and disposed at both sides of the membrane 4. It also has cell frames 20 disposed at the outside of the positive electrode 5 and at the outside of the negative electrode 6, respectively.
Each of the cell frames 20 has a bipolar plate 21 made of carbon plastic and a frame 22 formed around the outside of the bipolar plate 21.
The frame 22 has a plurality of holes which are called manifolds 23A, 23B. The manifolds 23A, 23B are arranged to form flow channels of the electrolytic solutions when a number of cells are stacked in layers and communicate with the conduit pipes 7, 8, 10, 11 of FIG. 6.
The redox flow battery is usually used with the aim of allowing load-leveling through the steady operation that electricity is discharged during daytime when more electric power consumption is required and electricity is charged (stored) during nighttime when less electric power consumption is required. For the load-leveling, high efficient operation of the battery is desirable from the viewpoints of energy saving and cost reduction. On the other hand, in case of emergency such as an instantaneous electric power failure in the steady operation, it is desirable to bypass the efficient operation in favor of highest possible overload operation of the battery. It should be noted here that the term “overload operation” means operation at an output in excess of a rated output, and the term “rated output” means an output at which energy efficiency during the charge/discharge of electricity reaches a design value or more. In general, the rated output is often set at about 80% of the maximum output.
The redox flow battery can allow a comparative high overload operation when it is in the fully charged state, but it cannot allow the overload operation substantially when electric energies stored in the electrolyte are less at the end stage of discharge or after the end of discharge.
This is because when the electrolyte is high in state of charge, the redox flow battery can allow a high overload output, while however, when the electrolyte drops in state of charge, the voltage is reduced, making it hard for the redox flow battery to allow the overload output. The expression “the electrolyte is high in state of charge” indicates the state that when a vanadium-based electrolyte is used for the electrolyte, the electrolyte for the positive electrode has a high ratio “(concentration of quinquevalent vanadium ions)/(concentration of tetravalent+quinquevalent vanadium ions)” and the electrolyte for the negative electrode has a high ratio “(concentration of bivalent vanadium ions)/(concentration of bivalent+trivalent vanadium ions)”.
For allowing this overload operation, the conventional redox flow batteries require a largely increased amount of electrolyte and also require that the electrolyte be constantly kept high in state of charge even after discharging in the steady operation. However, the load-leveling operation requires a fluid volume of electrolyte corresponding to its capacity of a few hours or more, and to obtain the constant increase in the state of charge by increasing the fluid volume of electrolyte requires a significantly large amount of electrolytes.
Accordingly, it is a primary object of the present invention to provide a secondary battery system that can allow a high overload operation even in the discharge state during the steady operation, and an operating method thereof.